-
Vol. 05 INTERNATIONAL JOURNAL OF PHOTOENERGY 2003
Spectroscopic studies of Ln(III) complexes withpolyoxometalates
in solids, and aqueous
and non-aqueous solutions
Stefan Lis,1,† Slawomir But,1 Andrzej M. Kłonkowski,2 and Beata
Grobelna2
1 Faculty of Chemistry, Adam Mickiewicz University, 60–780
Poznań, Poland2 Department of Chemistry, University of Gdańsk,
80–952 Gdańsk, Poland
Abstract. Chosen polyoxometalate (POM) anions and their
lanthanide(III) complexes, LnPOM, havebeen synthesized and
spectroscopically characterized in solid state, aqueous and
non-aqueous solutions.POMs, such as Keggin’s, Dawson’s and
Anderson’s type, Na9EuW10O36, compositions that function as
in-organic cryptands ([(Na)P5W30O110]14−-Preyssler anion, and
[(Na)As4W40O140]27−), containing inorganic(Na+,K+,NH+4 ) or organic
(tetrabutylammonium, NBu
+4 ) counter cations were obtained and their Ln(III) com-
plexes (sandwiched and encapsulated) studied. The synthesized
compounds were identified using elementaland thermogravimetric
analysis, UV-Vis spectrophotometry and FTIR spectroscopy. The
complexation stud-ies were carried out with the use Nd(III) and
Er(III) optical absorption and Eu(III) luminescence
spectroscopy.Luminescence characterization, including results of
intensity, quantum yields and luminescence lifetimes ofEuPOM
complexes in aqueous, non-aqueous solutions (DMF, DMSO,
acetonitryle) and solid are discussed.Based on luminescence
lifetime measurements of the Eu(III) ion the hydration numbers of
its sandwiched(efficient emitters) and encrypted complexes have
been determined and quenching effect discussed. TheEu(III)
complexes entrapped in a xerogel matrix have been studied as
luminescent materials. Luminescenceintensity, lifetime and quantum
yield of the EuPOM materials and their photochemical stability,
during con-tinuous UV irradiation, were tested.
1. INTRODUCTION
The majority POMs composed of molybdenum andtungsten
polyhedrons, due to their interesting physic-ochemical properties,
and biological importance, havebeen a subject of numerous studies.
This class of com-pounds has received much attention in the last
twodecades because of their wide applications in catalysis,material
engineering, photochemistry and in medicine.Their attractive and
often unusual physicochemicalproperties are related to their
structure, shape, chargedensity, redox potential, acidic character
and solubility.POMs with alkali counter cations such as Na+, K+
andNH+4 are well water-soluble compositions. They can besolved in
solvents of different polarity, depending onthe kind of the counter
cation occurring in the POMmolecule [1–4]. Generally POMs can be
categorizedinto three structural groups, depending on the
coordi-nation number of the heteroatom [1,2]. Our previousstudies
concerned synthesis of POMs and their Ln(III)complexes,
representing the three structural groups ofPOMs and additionally
so-called inorganic analoguesof crown-ethers and cryptands (e.g.
Preyssler’s anion).They have been characterized using
spectroscopicmethods both in aqueous [2,5–8] and
non-aqueoussolutions [9]. Some of LnPOM complexes show in-teresting
and effective luminescence properties, i.e.high luminescence
intensity, its quantum yields
† E-mail: [email protected]
and long lifetimes of Ln(III) excited states [10]. Suchcomplete
inorganic materials, particularly EuPOMcomplexes, have been
entrapped in xerogel matrices bysol-gel method and examined as
luminescent materi-als. The resulting immobilization of Eu(III)
complexesin xerogel matrices is known to enhance emissionintensity
[11, 12]. These luminescent materials havebeen also tested for
their photochemical stabilityunder continuous UV irradiation.
2. EXPERIMENTAL
2.1. Methods. All reagents used in these stud-ies were at least
analytical grade, while Nd2O3 andEu2O3 were spectroscopically pure
and non-aqueoussolvents (DMF, DMSO, acetonitryle,
dichloromethanefrom Fluka) pure for UV spectroscopy (content ofH2O
< 0.005%). Elemental analysis of the POMs andtheir europium
complexes were made with the use ofan Elemental Analyser 2400 CHN,
Perkin Elmer. Ther-mogravimetric (TG) and differential thermal
(DTA) anal-ysis were conducted using a Shimadzu
TGA-50H/A50thermoanalytic system, temperature interval was 293–823
K, heating rate 2 K/min in air atmosphere. Absorp-tion spectra were
recorded on a UV-2401PC Shimadzuspectrophotometer. The IR spectra
were obtained bymeans of FTIR Bruker JFS 113v spectrophotometer
forthe samples (∼2mg) prepared in KBr. The correctedluminescence
spectra of Eu(III) were recorded using
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234 Stefan Lis et al. Vol. 05
Table 1. The luminescence data of Eu(III) ion in EuPOM complexes
in solutions, λexc = 394 nm, CEu(III) = 0.001 mol/l.
CompoundLifetime
Hydration numberLuminescence Intensity [a.u.],
Quantum yield[ms] λmax = 615 nm
Eu(ClO4)3 0.11 9.3 0.01 2.60× 10−3Na9[EuW10O36] 2.29 0.2 2.0c
4.19× 10−3K17[Eu(P2W17O61)2] 3.08 0.1 0.4 1.38× 10−2Eu2TeMo6O24
0.19 5.1 0.08 –
K25[(Eu)As4W40O140] 0.25 3.7 0.02d 9.06× 10−3K12[(Eu)P5W30O110]
0.30 3.1 0.23e 9.6× 10−5(NBu4)12[(Eu)P5W30O110]a 0.79 0.6 0.37e
1.1× 10−4(NBu4)8H3[Eu(PMo2W9O39)2]b 1.11 0.2 0.95f –
K11,13[Eu((P/Si)MoxW11−xO39)2]1.2–3.5 0.1 0.0–4.8 10−4–10−2
Ref. [6] Ref. [6] Ref. [18]
a in acetonitryle, b in DMF, c λmax = 300 nm, d CEu(III) = 1×
10−4 mol/l, e CEu(III) = 1× 10−2 mol/l, f) λexc=342 nm.
Table 2. The luminescence characterization of EuPOM complexes
entrapped to xerogels.
HPAS Type of matrixLifetime [µs], Luminescence Intensity
[a.u.],
Quantum yieldλexc = 394 nm λmax = 615 nm
Na9EuW10O36 SiO2 642.8 23.7∗ 0.123SiO2 − PDMS 491.9 19.8∗
0.104SiO2 − TEG 508.4 24.4∗ 0.066
Eu2TeMo6O24 SiO2 245.9 15.3+ 0.027SiO2 − PDMS 162.9 23.8++
0.018SiO2 − TEG 275.4 37.6+++ 0.012
∗ λexc = 394 nm, + λexc = 320 nm, ++ λexc = 315 nm, +++ λexc =
330 nm.
a Perkin-Elmer MPF-3 spectrofluorimeter. The lumines-cence
lifetime of Eu(III) was measured with the use ofthe detection
system described earlier, consisting of anitrogen laser (KB6211)
and a tunable dye laser [13].The luminescence decay curves observed
in this workwere analyzed by a single exponential fitting,
provid-ing the decay constants. The luminescence quantumyield, Φ,
of the Eu(III) ion in solution was calculatedusing [Ru(bpy)3]Cl2 as
a standard. In the case of pow-der samples Φ was determined based
on the proceduredescribed by Wrighton et al. [14]. Measurement of
lu-minescence quantum yields of the powder samples in-volved
determination of the diffuse reflectance of thesample relative to a
nonabsorbing standard (KBr) at theexcitation length, and then
measuring the emission ofthe sample under the same conditions [15].
The quan-tum yield is the ratio of the emitted photons to
thedifference in the number of diffuse reflected photonsfrom the
sample and the nonabsorbing standard, hasbeen calculated using the
equation (1):
Φ = ERstd − Rsmpl
(1)
where E is the area of the corrected emission curveof the
sample, Rstd and Rsmpl are corrected areas un-der the diffuse
reflectance curves of the nonabsorbingstandard and samples,
respectively, at the excitation
wavelength. The calculated values of the luminescencequantum
yields of the samples studied are given in Ta-bles 1 and 2.
The Eu(III) complexes entrapped in xerogels wereirradiated by
means of a UV lamp of radiation power0.53 W/cm2.
2.2. Synthesis and identification of compounds.The lacunary
structures of the heteropolyanions (forexample SiW11O
8−39 ) were obtained from the corre-
sponding plenary structures (SiW12O4−40 ) as a result
of their partial degradation as described earlier [16].Europium
sandwiched complexes Eu(POM)2 were ob-tained according to the
method described by Pea-cock and Weakley [17] and modified by us
[18]. Thecryptand analogues of the heteropolyanions and
theirEu(III)-encrypted derivatives were prepared as previ-ously
described [5, 15, 19, 20]. Tetrabutylammoniumsalts of the chosen
compounds was obtained with us-ing a literature procedure [21].
Identification of syn-thesized compounds was done by comparison of
theIR spectra of obtained compounds with those previ-ously
reported, elemental and thermogravimetric anal-ysis [5–7, 18] and
our own spectrophotometric methodfor determination of tungsten and
molybdenum con-tents. The spectrophotometric method enables
thesimultaneous determination of W(VI) and Mo(VI) in
-
Vol. 05 Spectroscopic studies of Ln(III) complexes with
polyoxometalates … 235
POMs and their Ln/POM complexes with
disodium-1,2-dihydrobenzene-3,5-disulfate (Tiron), used as
colori-metric reagent [22].
2.3. Synthesis of the POMs entrapped in sol-gelmatrices. A wet
gel product was obtained after somedays by sol-gel process
(hydrolysis and polyconden-sation) of tetramethoxysilane (TMOS) in
mixture withmethanol and aqueous solution of EuPOM complex[23].
Methylated silicate xerogels was synthesized inthe similar way. A 1
: 1 mixture of TMOS (Aldrich)and polydimethylsiloxane (PDMS 200,
Aldrich)) was dis-solved in methanol and water as reagent was
added[24]. In the case of TMOS and triethylenglycol (TEG)with 1 : 1
molar ratio, the components were dissolvedin methanol, heated to 60
◦C and stirred for ten min-utes [25]. Then aqueous solution of
EuPOM complexwas added. In each gel preparation the molar
ratio[alkoxidegroup] : [H2O] = 1 : 1 and the resulting wetgels were
dried at room temperature to obtain xero-gels. The final
concentration of the Eu(III) complexesin xerogels are 5× 10−5 mol/g
xerogel.
3. RESULTS AND DISCUSSION
The compositions of POMs were determined based onresults of the
elemental (C, N, H), thermogravimetricanalysis (H2O, [NH4]+,
[NBu4]+, H+), spectrophotomet-ric determinations of Mo and W in the
range of UV-Vis,and FTIR spectra analysis. The elemental analysis
wasvery useful in the case of POM compositions containingan organic
counter cation.
The use of the FTIR spectroscopy has shown to be avery useful
tool in studies of POMs and their complexesdue to easy indication
of the plenary, lacunary and ofsandwich complexes, based on
characteristic featuresof the spectra [6]. The formation of the
lacunary struc-ture (pH dependent degradation of an appropriate
ple-nary structure) indicates a split of the P-Oa and W-Oc-Wband
oscillations, as a consequence of existence oftwo different
coordination environments of phospho-rus and oxygen atoms, Oc. FTIR
spectroscopy has beenused to fingerprint the POM structures because
of goodcorrelations between spectral peak positions, shapes,and
relative intensities of the spectra, obtained for solidand
solution. These correlations strongly indicate iden-tical
structures [18]. The FTIR spectra of tetrabutylam-monium
compositions of POMs have generally the samebands as the spectra
recorded for the potassium or am-monium salts. Additional bands
attributed to the C−Hand C− C oscillations of the (NBu4)+ organic
species,in the range of 1300–1500 cm−1 and 2800–3000 cm−1,occur in
this case.
Absorption spectra of Nd(III) and Er(III) in therange of their
hypersensitive transitions 4I9/2 → 2H9/2and 4I15/2 → 2H11/2,
respectively, were measured forvarious metal : ligand ratios in
non-aqueous solutions.Positions and intensities of the spectra are
sensitive to
the ligand field of POMs and can be used to evaluatethe
formation of LnPOM complexes. Examples of thespectra, measured for
various Ln(III) : POM molar ra-tios, ranging from 1 : 0 to 1 : 3,
in DMSO and DMFsolutions, are presented in Figures 1 and 2.
560 570 580 590 600 610A
bso
rban
cenm
0.00
0.02
0.04
0.06
0.080.08
0.06
0 1 2 3CHPAS/CNd(III)
Nd : POM1 : 01 : 0.41 : 0.71 : 1.01 : 1.41 : 1.81 : 2.01 :
2.5–3.0
Figure 1. Absorption spectra of Nd(III) in the rangeof the
hypersensitive transition (4I9/2 → 2H9/2) for var-ious Nd :
[NBu4]6H[PMo2W9O39] molar ratios in DMSO,CEr(III) = 0.001
mol/l.
510 520 530
nm
Ab
sorb
ance
0.00
0.02
0.04
0.04
0.020 1 2 3
CHPAS/CEr(III)
λmax=521 nm
Er(III) : POM1 : 01 : 0.41 : 0.71 : 1.01 : 1.41 : 1.81 : 2.01 :
2.51 : 2.8
Figure 2. Absorption spectra of Er(III) in the range of
the hypersensitive transition (4I15/2 → 2H11/2) for var-ious Er
: [NBu4]6H[PMo2W9O39] molar ratios in DMF,CEr(III) = 0.001
mol/l.
These spectra show an increase in absorption and ashift of their
maxima consistent with formation of ML2complexes.
Recently, based on the analyses of the absorptionspectra of
Nd(III) and Er(III) ions, we evidenced for-mation of the ML and ML2
complexes with Keggin’sand Dawson’s type of polyanions [7, 18, 26]
and MLcomplexes with the polyanion [(Na)As4W40O140]27−
[18] in aqueous solutions. In the case of [MnMo9O32]6−
-
236 Stefan Lis et al. Vol. 05
(Anderson’s anion) with the Nd(III) ion spectroscopicstudies
demonstrated that the M2L type of complexesoccur in solution
[7].
Formation of the ML2 sandwiched complexes innon-aqueous solvents
(DMSO, DMF, acetonitryle) asare shown in Figures 1 and 2. They were
also ear-lier observed in aqueous solutions [7, 26]. Our stud-ies
concerning chemometrics and factor analysis of theNd(III)
absorption spectra with chosen POM complexesconfirmed formation of
the M2L, ML, ML2 complexes[26, 27]. In spectrophotometric studies
we also provedthat formation of LnPOM complexes is function of
ionicstrength. For example, in equimolar solutions of Nd(III)and
Keggin’s POM of high ionic strength, a preferentialformation of the
sandwich complexes Nd(HPAS)2, in-stead of NdHPAS, were observed [6,
7].
Eu(III) luminescence studies (intensity, lifetimemeasurements
and quantum yield) of the EuPOM sys-tems were used to investigate
their emission proper-ties, effectiveness of energy transfer
processes, hydra-tion numbers and complex compositions [6, 7].
The Eu(III) luminescence lifetimes measured for theEu/POM
complexes were used to calculate the numberof water molecules
present in the inner sphere of theEu(III) ion, using the following
eq. [28]:
nH2O = 1.05τ−1H2O − 0.7 (2)
The lifetime of Eu(III) excited state is efficientlyquenched by
OH oscillators of inner sphere H2Omolecules bound to the Eu(III)
ion. Based on the Eu(III)luminescence lifetime measured for EuPOM
complexesin non-aqueous solutions and solid the hydration num-bers
of this ion were calculated as shown in Table 1and compared with
those obtained earlier in aque-ous solution [5–7]. The measured
Eu(III) luminescencelifetime gave the longest values for sandwiched
com-plexes with inorganic counter cations in aqueous so-lution
(from 1.2 to 3.5 ms) and with organic countercation (1.11 ms) in
DMF solution. The hydration num-bers calculated from the measured
lifetime values in-dicate no water molecules in the Eu(III) inner
coordi-nation sphere of the complexes. In the case of
Eu(III)encapsulated Preyssler’s complexes, the compositionwith
organic counter ion (NBu4)12[(Eu)P5W30O110] hassmaller hydration
number (∼ 0.5) than that with inor-ganic counter ion
K12[(Eu)P5W30O110] having 3 watersof hydration. It is worth to
mention that the solid Eu(III)complexes have coordination number of
8 or 9 in thecase of solution.
Examples of the luminescence excitation and emis-sion spectra of
Eu/POM complexes recorded in non-aqueous solvents are presented in
Figure 3.
The Eu(III) luminescence lifetimes obtained forEu/POMs in
non-aqueous solvents with various molarratios of the components
demonstrated the ML2 com-plex formation (see Figure 4).
250 300 350 400 560 600 640 680 7200.0
0.5
1.020000
10000
0
nm
Lum
ines
cen
ceIn
ten
sity
[a.u
.]
(a) (b)
Lum
ines
cen
ceIn
ten
sity
[a.u
.]
Figure 3. Luminescence excitation (a) and emission (b) spec-
tra of Eu(III) in (NBu4)8H3[Eu(PMo2W9O39)2] in DMF.
0 1 2 3 4CPOM/CEu(III)
CEu(III) = 0.002 mol/l(NBu4)5H2[PW11O39]
(NBu4)6H[PMo2W9O39]
Lum
ines
cen
ceLi
feti
mes
[µS]
500
1000
1500
Figure 4. Eu(III) luminescence lifetime as a function of thePOM
: Eu(III) molar ratios in non-aqueous solvent (DMF).
The luminescence spectra of non-aqueous solu-tions are generally
similar to those obtained in aque-ous solution [6]. Excitation
spectra of the Eu(III)molib-dotungstate compounds show strong
emission band(λmax ∼ 340 nm) due to ligand to metal energy
trans-fer (LMET) from the tungstate group to the Eu(III) ion[29,
30].
Our previous studies have shown that the mostintense Eu(III)
luminescence was observed for the(EuW10O36)9− and
[Eu(SiMoW10O39)2]13− sandwichedcomplexes due to energy transfer
from the tungstategroup to the Eu(III) ion. In the case of
[Eu(SiW11O39)2]13−
and [Eu(P2W17O61)2]17− and the Eu-encrypted com-plexes a weak
luminescence intensity was observed. Inlatter cases the transfer
from ligand to Eu(III) does notoccur. Interesting pattern of the
Eu(III) luminescencelifetime and quantum yield were observed in the
case ofthe heterotungstomolybdate Eu-sandwiched
complexes{[Eu(Si(P)MoxW11−xO39)2]13−}. A linear dependence
-
Vol. 05 Spectroscopic studies of Ln(III) complexes with
polyoxometalates … 237
0 50 100 150 200 250 300
Exposure Time [min]
Emis
sion
Inte
nsi
ty[a
.u.]
0
10
20
30
40
50
60
70
Na9EuW10O36 ·19H2O in SiO2, conc.=5·10−5 mole/g SiO2
Eu2TeMo6O24 ·18H2O in SiO2, conc.=5·10−5 mole/g SiO2
Figure 5. The photochemical stability of EuPOMs com-
plexes.
of the Eu(III) luminescence lifetime, τ , and quantumyield, φ,
on the content of Mo (number of atoms x,where x = 0–9) in the
[Eu(Si(P)MoxW11−xO39)2]13−structures were observed. These
dependences can beapplied for the determination contents of Mo in
poly-tungstomolybdate complexes [6, 9, 18].
Based on luminescence lifetimes measured both forsolid and
aqueous solutions, we found no H2O in Eu(III)inner sphere in the
sandwiched complexes Eu(POM)2.Whereas the complexes Eu2POM have
four or six watermolecules and europium-encrypted derivatives
possessthree or four H2O’s in the Eu(III) inner
coordinationsphere.
In order to increase photochemical stability andto minimise
water (O−H oscillators) interaction, weused Eu(III) complexes with
the heteropolyanions(Na9EuW10O36,Eu2TeMo6O24) entrapped in silica
xe-rogel matrix. The luminescence characterization ofEuPOM
complexes entrapped to xerogels are presentedin Table 2. The
resulting encapsulation and immobiliza-tion of Eu(III) complexes
with POMs in xerogel matricesenhanced emission intensities and
luminescence quan-tum yield related to solid EuPOM and EuPOM
dissolvedin the solutions.
The Eu(III) complexes entrapped in xerogels weretested for their
photochemical stability during UV ir-radiation. As it is shown in
Figure 5, the emission in-tensity (at λmax = 617 nm, when λexc =
273 nm) of thesilica xerogel doped with Eu(III) complexes during
UVirradiation remains constant within experimental error.Thus, for
the Eu(III) in inorganic environment there ispresent no
photodegradation effect due to relative highenergy (UV) quanta used
for the irradiation.
ACKNOWLEDGEMENT
This work was supported by the Polish State Committeefor
Scientific Research, Grant No. 4 T08A 044 23.
REFERENCES
[1] M. T. Pope, Heteropoly and
Isopolyoxometalates,Springer-Verlag, New York 1983.
[2] S. Lis, Acta Phys. Pol. A 90 (1996), 275.
[3] D. E. Katsoulis, Chem. Rev. 98 (1998), 359.
[4] M. T. Pope and A. Müler, Angew. Chem. Int. Ed.Engl. 30
(1991), 34.
[5] S. Lis, M. Elbanowski, and S. But, Acta Phys. Pol. A90
(1996), 361.
[6] S. Lis and S. But, Materials Science Forum 315–317(1999),
431.
[7] S. Lis and S. But, J. Alloys Comp. 300–301 (2000),370.
[8] A. Szyczewski, S. Lis, Z. Kruczynski, and S. But, J.Alloys
Compd. 341 (2002), 307.
[9] S. But, Ph.D. Thesis, Adam Mickiewicz University,Poznan,
Poland, 1999.
[10] R. Ballardini, E. Chiorboli, and V. Balzani, Inorg.Chim.
Acta 95 (1984), 323.
[11] O. A. Serra, E. J. Nassar, G. Zapparolli, and I. L. V.Rosa,
J. Alloys Compd. 207–208 (1994), 454.
[12] V. Bekieri, G. Pistolis, and P. Lianos, J. Non.
Cryst.Solids 226 (1998), 200.
[13] Z. Stryla, S. Lis, and M. Elbanowski, Optica Appli-cata
XXIII (1993), 163.
[14] M. S. Wrighton, D. S. Ginley, and D. L. Morse, J.
Phys.Chem. 78 (1974), 2229.
[15] A. M. Klonkowski, S. Lis, M. Pietraszkiewicz, Z.Hnatejko,
K. Czarnobaj, and M. Elbanowski, Chem.Mater. 15 (2003), 656.
[16] Y. Jeannin and J. Martin-Frere, Inorg. Synth. 27(1990),
71.
[17] R. D. Peacock and T. J. R. Weakley, J. Chem. Soc. A(1971),
1836.
[18] S. Lis and S. But, J. Inclusion Phenom. Molec. Recog-nition
Chem. 35 (1999), 225.
[19] I. Creaser, M. C. Heckel, R. J. Neitz, and M. T.
Pope,Inorg. Chem. 32 (1993), 1573.
[20] M. R. Antonio and L. Soderholm, Inorg. Chem. 33(1994),
5988.
[21] J. Bartis, S. Sukal, M. Dankova, E. Kraft, R. Kronzon,M.
Blumenstein, and L. C. Francesconi, J. Chem.Soc., Dalton Trans. 11
(1997), 1937.
[22] S. Lis and S. But, J. Alloys Compds. 303–304
(2000),132.
[23] K. Czarnobaj, M. Elbanowski, Z. Hnatejko, A. M.Klonkowski,
S. Lis, and M. Pietraszkiewicz, Spec-trochim. Acta A 54 (1998),
2183.
[24] Z. Hnatejko, A. M. Klonkowski, S. Lis, K. Czarnobaj,M.
Pietraszkiewicz, and M. Elbanowski, Mol. Cryst.Liq. Cryst A. 354
(2000), 207.
[25] P. Judeinstein and H. Schmidt, J. Sol-Gel Sci. Tech.3
(1994), 189.
[26] G. Meinrath, S. Lis, S. But, and M. Elbanowski, Ta-lanta 55
(2001), 371.
-
238 Stefan Lis et al. Vol. 05
[27] G. Meinrath and S. Lis, Fresenius, J. Anal. Chem.369
(2001), 124.
[28] P. P. Barthelemey and G. R. Choppin, Inorg. Chem.23 (1989),
2044.
[29] G. Blasse, G. J. Dirksen, and F. Zonnevijlle, J.
Inorg.,Nucl. Chem. 41 (1981), 2947.
[30] G. Blasse, Eur. J. Solid State Inorg. Chem. 28
(1991),719.
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